The present invention relates to a method of airtight bonding of two membranes.
The problem of bonding two membranes together in such a way as to minimize the space remaining between the membranes and seal it with respect to the medium surrounding the membranes is known from the field of pressure sensors, for example. German Patent No. 4 419 593 A1 discloses a pressure sensor arranged in a pressure measurement chamber suitably positioned in the tube or channel through which a liquid or gas is flowing. The pressure measurement chamber has an opening in which a membrane is stretched evenly by means of a tension device and is deflected according to the pressure prevailing in the flowing fluid. In the area of the membrane affected by pressure fluctuations in the fluid, a pressure transmitting part also composed of a membrane is arranged to respond to the movements on the membrane on which the pressure is. acting. Airtight bonding of the two membranes is accomplished by pressing them at their edge areas so as to minimize the space remaining between the membranes.
European Patent No. 130 441 describes a device whereby a membrane on which pressure is applied is completely bonded to a membrane of a pressure measurement device by exhausting the air between the two membranes by using a vacuum pump. Such a device makes it possible to measure overpressures and negative pressures due to the tight contact between the two membranes, but it has the disadvantage that means must constantly be provided for creating a vacuum in order to guarantee reliable operation of the device.
The object of the present invention is to make available a simple method of airtight and complete bonding of two membranes.
This object is achieved according to the present invention by the fact that the method includes the steps of producing a convex curvature in one of the membranes, placing the membrane having the convex curvature on the membrane to be bonded to it in an airtight manner, joining the membranes until the edge areas of the membranes are in contact and applying a pressing force that seals the membranes with an airtight seal in the edge area. Such a method makes it possible to completely displace the air between the two membranes and prevent the admission of air. This is achieved by pressing the convex curvature of the bulging membrane into the other membrane when they are joined, thereby gradually displacing the air between the two membranes. The joining of the membranes is performed until the membranes are ultimately in contact even in their edge areas. To prevent admission of air from the area surrounding the membranes, a pressing force is applied according to the present invention after the membranes are joined, causing the membranes to be sealed airtight in their edge areas. This yields the advantage that an airtight connection as well as a complete bonding of the membranes is achieved, resulting in a reliable bonding when there is an overpressure or a negative pressure, and thus providing synchronized movement of the membranes.
In contrast with planar contact between two membranes, the convex curvature of one of the membranes achieves the result that the possibility of an air cushion remaining in the area between the membranes after the edge area has been joined and pressed together is ruled out. According to the present invention, the air is displaced between the membranes due to the fact that at first the joining of the two membranes does not take place over the entire cross-sectional area but instead at the beginning of joining only in a protruding area of the membrane created by the curvature.
In another embodiment of the present invention, a first membrane borders a first chamber of a membrane unit of a metering pump to accommodate a working fluid, and a second membrane borders a second chamber to accommodate a fluid that is to be delivered, with the convex curvature in the first membrane being produced by increasing the pressure on the working fluid in the first chamber. It is thus possible to join the membrane of a pump head of a diaphragm pump in an airtight and complete manner to the membrane of the pump bordering the working fluid. To that effect, the pressure of the working fluid is increased slightly due to the movement of a piston, for example, thus resulting in a convex curvature of the membrane bordering the working fluid. The membrane of the pump head is joined to the convexly curved first membrane by the method according to the present invention, after which the bulging membrane is placed on the membrane of the pump head to which it is to be joined, both membranes being joined together until their edge areas are joined, whereupon a pressure force is applied, sealing the membranes and making them airtight in the edge area.
It is especially advantageous if the second chamber is filled with a fluid before joining the two membranes, and if it is emptied completely or partially by the joining of the membranes. The second chamber may be filled with a liquid or, for example, a gas under pressure which is forced out of the second chamber during the joining in accordance with the convex curvature of the first membrane.
In a further embodiment of the present invention, the two chambers are positioned and joined by a guide. In particular, this facilitates precise placement of the bulging membrane onto the membrane to which it is to be joined with an airtight seal in the desired position. Furthermore, joining the two chambers with the help of a guide further ensures that the edge areas of the membranes will be in precise contact so that this will ensure that an airtight connection is established in the entire edge area of the membranes.
In another embodiment of the present invention, it is possible for each chamber to be arranged in a housing part and to be held together by a tension device arranged on the housing parts with the desired pressure after joining. This can ensure that a pressure adequate to achieve an airtight connection will be applied to the edge areas of the membranes.
The curvature of one of the membranes can be increased while joining the two membranes. Thus, a displacement of the air between the two membranes and an increase in the contact area between them are achieved not only by joining them but also by increasing the curvature of one of the membranes. For example, if one membrane of one diaphragm pump head is to be joined to the membrane of the diaphragm pump bordering the working fluid, then the pressure may be increased in the working fluid after the membrane that already has a curvature has been positioned and during the joining of the two membranes so that the convex curvature in the first membrane is increased.
In a further embodiment of the present invention, the curvature in the membrane is designed in such a way that the membrane to be joined to it is mostly or completely deflected after the two membranes have been joined. What is achieved in an especially reliable manner is that air between the two membranes is displaced before the edge areas of the membranes are joined together. The convex curvature of the membrane may be formed completely during the positioning of the membrane or it may be increased during the joining.